1. Field of the Invention
The present invention relates to a plane display and more particularly to a highly reliable liquid crystal display.
2. Description of the Related Art
A liquid crystal display features low power consumption and a small thickness of its body and is gaining widespread applications. An active matrix type liquid crystal display having pixels in the form of thin film transistors has good picture quality and is considered to be the leading configuration in this technical field. However, from the standpoint of reliability, this type of liquid crystal display suffers from degradation in picture quality. More particularly, electrical resistance of the liquid crystal changes overtime degrading in characteristics, and/or leakage current in switching transistors is increased with time, with the result that a predetermined level of voltage can not be maintained and hence transmitivity of the liquid crystal is changed. This causes a degraded picture quality such as degradation in contrast. To cope with this problem, a design has been proposed in which storage capacitors are provided in association with respective pixels. An example of this design is described in SID 84 DIGEST, 1984, pp. 312-315. Plan and sectional views of a specified construction of the storage capacitor are shown in JP-A-61-13228.
FIGS. 10 and 11 illustrate the fundamental construction of the prior art storage capacitor. Referring to these figures, a thin film transistor 2 provided in each pixel has a source 14 connected with a pixel transparent electrode 12, an insulating film 18 lies beneath the electrode 12, and a transparent electrode 13 of a storage capacitor lies beneath the insulating film 18 and extends to an end of a matrix substrate 1. At the matrix substrate end, the electrode 13 connects to a storage capacitor conductor 8 which extends to an external terminal. The pixel transparent electrode 12 and storage capacitor transparent electrode 13 may both be made of indium tin oxide (ITO). A signal voltage on a signal line 4 is transmitted from a drain 16 to the source 14 by applying a gate voltage to a gate 15 connected to a scanning line 3 to turn on the transistor 2. Then, the pixel transparent electrode 12 electrically connected to the source 14 drives liquid crystal to activate a capacitor across the liquid crystal and at the same time cooperates with the storage capacitor electrode 13 to form, across the insulating film, a storage capacitor of a capacitance which is a few times the value of the capacitance of the liquid crystal capacitor. The storage capacitor is effective to retain the voltage applied across the liquid crystal for a predetermined period of time.
The above construction was applied to relatively compact liquid crystal displays of 3-inch to 5-inch diagonal and its meritorious effects have been corroborated. On the contrary, large-sized liquid crystal displays face a new problem. More particularly, as the size of display increases to about 10 inches or more in diagonal, the storage capacitor electrode 13 connecting by itself to the conductor 8 exhibits an increased line resistance inside the display region and as a result a sufficient amount of charge can not be stored in the storage capacitor within the predetermined scanning time. This impairs the effects of the storage capacitor.
As is clear from the foregoing, the prior art display fails to consider a potential drop developing in the storage capacitor transparent electrode inside the display region of large-screen displays and it faced has the problem that it can not give full play to its storage capacitor effects.